Microsecond Electroporator Optimization for Parasitic Load Handling and Damping

During development of the pulsed power generators the occurrence of the parasitic elements in the circuit is inevitable, which leads to appearance of overvoltage, overcurrent and waveform distortions. This work is focused on the optimization of the microsecond electroporator to enable handling and damping of the parasitic loads. The optimization is based on a flexible PSPICE model of the electroporator, which is used for the compensation of the parasitic parameters. Based on the modelling results the parameters and the circuit elements for the device are selected. The compliance of the prototype’s experimental and the PSPICE simulated output pulses is analysed. The optimized circuit of the microsecond electroporator is designed. The system supports current handling up to 100 A and capable to generate up to 4 kV square wave pulses. DOI: http://dx.doi.org/10.5755/j01.eee.21.6.13758

1 Abstract-During development of the pulsed power generators the occurrence of the parasitic elements in the circuit is inevitable, which leads to appearance of overvoltage, overcurrent and waveform distortions.This work is focused on the optimization of the microsecond electroporator to enable handling and damping of the parasitic loads.The optimization is based on a flexible PSPICE model of the electroporator, which is used for the compensation of the parasitic parameters.Based on the modelling results the parameters and the circuit elements for the device are selected.The compliance of the prototype's experimental and the PSPICE simulated output pulses is analysed.The optimized circuit of the microsecond electroporator is designed.The system supports current handling up to 100 A and capable to generate up to 4 kV square wave pulses.Index Terms-Electroporation, pulsed power, stray inductance, compensation.

I. INTRODUCTION
Electroporation is a biomedical technique, which is applied for the targeted and controlled delivery of drugs and other molecules into the cells by means of increase of the cell membrane permeability [1]- [3].
The high voltage generators are required for the high intensity electric field generation, which induces reversible and non-reversible cell permeability increase effects [4]- [6].As a rule a controlled discharge of a capacitor array through the biological load using a pulse forming switch is applied [7].However, when the load is increased and the commutated voltage is in the range of several kilovolts, the influence of the stray inductance and other parasitic circuit components is inevitable [8].Appearance of transient overvoltage and overcurrent can damage the switch, distort the output waveform and introduce safety issues during operation of the device.The design and implementation of the compensation circuits for parasitic load handling and damping is required [9]- [10].
Compensation of the parasitic elements involves limitation of the circuits' dI/dt and dV/dt [11]- [12].Damping of the reverse biased voltage and current is also required.However, implementation of the compensation circuit without preliminary analysis of the transient processes could result in an over-damped or under-damped response, which is in both cases undesirable and may result in irreversible damage of the electroporator or poor pulse generation flexibility.
In this work we present a flexible microsecond electroporator optimization using PSPICE model of the device, which can be used to estimate the influence of transient processes during pulse generation and allows design and implementation of the compensation circuit for parasitic load handling and damping.Optimization of the 4 kV, 100 A electroporator based on IXEL40N400 IGBT switch is performed.The adequacy of the proposed model is evaluated experimentally.

II. DEVELOPMENT OF THE COMPENSATION CIRCUIT
As it has been mentioned above the conventional way of generation of the square-wave electrical pulse is the discharge of the capacitor array through the load using a controlled opening time switch.In this study an insulated gate bipolar transistor (IGBT) switch is applied.The IGBT is the best choice in the micro-millisecond diapason due to the relatively fast rise and fall times and the high power handling capability compared to the metal-oxidesemiconductor field-effect transistor (MOSFET).
In order to determine the influence of the parasitic components and develop a compensation circuit for the microsecond electroporator a PSPICE model has been introduced.The PSPICE modeled circuit is shown in Fig. 1.The model consists of the voltage source that charges the main capacitor bank CB, IGBT model, the load and the parasitic components that are expected in the circuit.Essentially, the IGBT switch is a MOSFET with additional layer in series with the collector, which implies that it could be simulated as a power MOSFET1 and a bipolar PNP transistor BJT1.The voltage breakdown features of the simulated switch are introduced using diodes D2 and D3.The stray capacitances, and the IGBT inner resistances are also simulated as CGC, CGE, CCE and the RC, RE, RG, respectively.The parameters of the IGBT have been altered to match the datasheet switching characteristics of the IXEL40N400 IGBT switch by Ixys (USA).
The IGBT is driven by an external signal, while the resistor RLIM limits the driving signal power.The LGS is the stray inductance of the driver circuit.The load of the electroporator is a cuvette with cell suspension.The load has been simulated as a resistor RLOAD.The parasitic components of the load such as contact capacitance, resistance of the transmission line and the stray inductance have been introduced as CLOAD, RLINE, LLINE, respectively.The summary of adjustable model parameters is presented in Table I.The pulse forming capabilities of the simulation circuit have been evaluated.In order to determine the required compensation for the transient processes during switching the influence of the parasitic circuit parameters LLINE and CLOAD have been analysed.The dependence of the generated 2 μs pulse waveform on the stray inductance LLINE is shown in Fig. 2.
As it can be seen in Fig. 2 due to parasitic inductance in the circuit the aberration from the square waveform is observed.The maximum allowed load of 40 Ω has been used in the model to estimate the maximum loop effect, which may appear in the circuit, during high current flow.The rise time of the pulse is increased dramatically to 0.5 μs-0.7 μs dependent on the stray inductance value.The switching delay of 200 ns-500 ns that is introduced in the simulation is typical for IGBTs, therefore was also implemented in the model to improve the adequacy of results.The voltage spikes and oscillations during turn-off of the IGBT are proportional to the total loop inductance of the electroporators' transmission line, which in the real prototype comes from PCB tracks, terminal leads, inner electronic component inductance and wire-bond connections.Combined with the self-inductance in high power electronic components the minimum total expected value of inductance is in the 2 μH-7 μH range.The wire connections contribute as much as <10 % of the total expected circuit inductance, however minimizing this value is still advantageous.
Another non-desired parameter influencing the output pulse shape is the parasitic capacitance.This parameter mainly impacts the rise time of the pulse during turn on of the high voltage IGBT.During turn-on there is a risk of high inrush current forming due to the charging of the stray capacitance, however it can be diminished by a ballast resistor in series with the load.In order to estimate the effect of the stray capacitance on the output pulse of the generator without compensation circuit the CLOAD parameter influence has been investigated in the model and is presented in Fig. 3.
A voltage spike below 10 % of the total pulse amplitude could be acquired when the 800 pF stray capacitance is present in the circuit.However, if the total stray inductance of the circuit will be higher than 2 μH or a series to the load ballast resistor is implemented, the effect of stray capacitance according to the proposed model is not significant and could be neglected.
The IGBTs have a significantly lower on state voltage compared to the MOSFETs, the high current and power handling capability and the high breakdown voltage limit makes it a better fit for the proposed application.However, one of disadvantages of the IGBT that will influence the output pulse of the generator is the tail current during turn off, until the charge carriers are swept out and recombined in the semiconductor structure of the switch.The influence of the tail current could be minimized by increase of the gate current, however the risk of the IGBT latchup increases.If the driving signal power is outside of the datasheet rating, the IGBT can fail and no longer be controlled by the transistor's gate.According to the datasheet, the optimal RGS for the IGBT is in the range of 30 Ω.The current tail of the IGBT has been evaluated using the model at maximum load and in the 0 kV-4 kV voltage range.The results of the simulation are presented in Fig. 4. As it can be seen in Fig. 4 up to 0.5 μs-1 μs increase of the fall time could be observed due to the tail current of the IGBT.Decrease of the gate resistance below the recommended value may involve safety issues due to the possible IGBT failure, therefore the minimum value of 30 Ω has been selected.One of the most important tasks during development of the electroporator is to protect the pulse forming switch from overvoltage and overcurrent.The oscillations occur due to the reverse biased current generated in the circuit by the parasitic inductive loop.Especially, the voltage spikes are experienced on the IGBT switch during transient state of turn-off.In order to prevent overvoltage issues on the IGBT, the switch transient processes dependence on the stray inductance of the circuit was investigated.The voltage spikes on the switch have been simulated with a 40 Ω load with various stray inductance in the circuit and are shown in Fig. 5.
As it can be seen in Fig. 5 the overvoltage protection circuit will be required in the prototype.The reverse biased voltage can reach up to 36 % of the total pulse amplitude if the stray inductance is in the 10 μH range.In the proposed application of the IXEL40N400, there is no reserve in the collector-emitter voltage, which implies that the generated voltage during transient processes is outside the applicable ratings of the switch if a 4 kV pulse is formed.Also reversebiased voltage will discharge the capacitor bank CB more rapidly, which is not desirable for repetitive pulsing.
In order to compensate the transient processes and improve the generated pulse shape an overvoltage protection circuit was developed, which includes RCD snubber and crowbar diode topology (DC1 and DC2).The circuit is shown in Fig. 6.The crowbar diodes damp the induced current through the load.The snubber diodes DS1 and DS2 prevent reverse biased current flow to discharge the capacitor bank CB.Also the DS2 prevents the reverse biased voltage spikes on the switch.RC snubber is implemented to limit the dI/dt in the circuit and have a reserve in time for the recovery of the snubber diodes.The RS1 = 1 Ω, while the optimal value for the CS1 has been found using simulation results, which are presented in Fig. 7.
As it is apparent from Fig. 7 the reverse biased voltage during transient processes due to the parasitic parameters of the electroporator circuit has been completely compensated even when the high inductance value of 10 μH is used.Both the switching dynamics of the IGBT and the shape of the output pulse have no oscillations or voltage spikes.However, there is still a risk of a spark occurring in the cuvette during electroporation, which implies that the value of CS1 equal to 5 nF-10 nF is preferable, ensuring a considerable reserve of dI/dt and dV/dt limitation.Based on the proposed PSPICE model the pulse forming unit of the electroporator has been developed, however certain trade-offs on the rise and fall times have been made, which negatively affected the minimum generated square wave pulse duration (5 μs).The prototype of the electroporator has been made.In order to estimate the adequacy of the model to simulate the experimental output pulse parameters the simulation data has been compared to the measurement results in the 5 μs-30 μs pulse range, when the duration of the rise and fall time is considerable compared to the total pulse width.

III. EXPERIMENTAL RESULTS
The typical output pulses of the developed electroporator with parasitic load compensation and dampening circuit have been compared to the simulation data of the model and are shown in Fig. 8.The pulse has been divided into three regions: turn on transient (10 %-90 % pulse amplitude), turn off transient (90 %-10 % pulse amplitude) and the pulse plateau (90 %-100 % pulse amplitude).The simulation uncertainties for the turn on and the turn off transients were evaluated on the time scale, while in the plateau case the difference in amplitude was investigated.Taking into account the 1.5 % DC accuracy variations of the oscilloscope and the 1 % resistance value uncertainty, the simulation model uncertainty in the plateau region was < ±3 %.On the time scale due to the high accuracy of the oscilloscope (±0.4 ns), the influence of the measurement equipment has been neglected.The time scale uncertainties of <±1.2 % for the turn on transient and the <±5 % for the turn off transient across the whole diapason of the generated pulses have been ensured.It has been confirmed that the simulation results are in acceptable compliance with the experimental data.

Fig. 2 .
Fig. 2. The influence of the stray inductance on the generated pulse waveform.

Fig. 3 .
Fig. 3.The influence of the stray capacitance on the generated pulse waveform.

Fig. 4 .
Fig. 4. The influence of the IGBT current tail on the pulse waveform.

Fig. 7 .
Fig. 7. Switching dynamics and the output pulse shape dependence on snubber capacitance.
IV. CONCLUSIONS A flexible PSPICE model for compensation of the transient processes in the electroporator circuit has been proposed.Based on the simulation results a parasitic load handling and dampening support has been implemented in the electroporator circuit.The simulation results are in acceptable compliance with the experimental data.The developed electroporator protototype generates 5 μs-10 ms square wave electrical pulses up to 4 kV.

TABLE I .
PSPICE MODEL PARAMETERS.